Liquid chromatography involves a process for separating various components of liquids in order to analyze the chemical content of the liquid. An amount of solute sample containing unknown components to be analyzed is injected into a chromatographic column. A carrier fluid is then introduced into the column under pressure, and causes the components of the sample to travel through the column at slightly different velocities. This results in the formation of essentially pure fractions of the components in bands or zones. The identity of the chemical components can be determined from measuring the separated bands using a spectrophotometer or other identifying device. Such chromatographic analyses are typically performed under high pressure conditions, usually 1,000 psig or higher, for greater speed.
The sample solution may be delivered to a chromatograph or other analyzer using a sampler-injector or autosampler. One type, such as shown in U.S. Pat. No. 3,918,913, delivers the sample by positive displacement of the sample from a vial to the analyzer. The sample is placed in a vial which is then closed by a plastic cap. At the proper time, a hollow needle is driven through a septum in the plastic cap, and a collar surrounding the needle forces the cap into the vial in piston-like fashion, pressurizing its contents and forcing the sample liquid through the needle and along a liquid line to the analyzer. Another type of sample delivery system uses negative displacement; for example, the sample may be withdrawn by a syringe. This method may be advantageous when only a small amount of sample is available.
Prior to injecting a sample into a chromatographic column for analysis, it is desirable to filter the sample to remove any particulate matter or other solid material which may be in the sample. However, filtration generally results in the loss of some of the sample within the filter material. If the sample is costly or available in very small amounts, it may be necessary or desirable to be able to conduct chromatographic analysis using a small amount of the sample, for example less than 1,000 microliters. It is sometimes necessary to use as little as one microliter drawn from a total volume of five microliters. Furthermore, a filtration step takes time, and can slow down total analysis time. Therefore, filtration should be highly efficient in order to minimize delays and loss of sample. Prior filtration devices which have been designed and used for filtering samples prior to chromatographic analysis have demonstrated problems in effectively and efficiently preparing a filtered sample in an amount sufficient for chromatographic analysis.
Prior manual sample filtration devices include syringes with filters mounted at the end of the syringe. The sample would be injected into a vial as it was being filtered, and then the vial could be used in an autosampler. Manual piston-type filtration devices are also known in the art, for example as shown in U.S. Pat. Nos. 3,846,077 and 3,955,423 issued to Ohringer, 4,891,134 issued to Vcelka, 4,895,808 issued to Romer, and 3,962,085 issued to Liston. If any such devices were used to prepare a sample for chromatographic analysis, it would be necessary to transfer the sample to an injection device. Some of the above prior art devices are open tubes which provide no protection against contamination of the sample between filtration and transfer. Others provide a removable cover, but the process of uncovering the sample and manually pouring it introduces the opportunity for human error or sample contamination. Labor costs are also higher when filtration and transfer are conducted manually. Inefficiencies increase if the device is reusable rather than disposable after a single use.
Therefore, it is highly desirable to filter the sample as part of the operation of an autosampler which injects the sample into an analyzer. U.S. Pat. No. 4,644,807, issued to Mar, discloses a sample delivery apparatus for delivering samples of liquid to chromatographic columns. A special sampling tip, designed specifically for use in the Mar device, fits into a recess in a plunger that is slidably mounted in a vial containing the sample. The special sampling tip is moved downward to force the plunger into the vial and thereby force the sample through a filter mounted in the plunger into the sampling tip. One problem with the Mar device is that only the porous filter material protects the sample from the environment while awaiting operation of the sampling tip. Gaseous contaminants might penetrate the filter, or solid particles might collect on the filter and be carried to the analyzer. For the same reason, the Mar device is not suitable for use with organic samples, which can evaporate through the porous filter material.
Another problem with the Mar device, and many other filtration devices which are known in the art, has been the relatively large bulk and thickness of the filter medium itself. When a filter is positioned across a fluid path and will be subjected to fluid under pressure, as in the positive displacement Mar device, it must have sufficient internal strength to avoid bursting under the pressure. The required strength has often been provided by making the filter thicker. As noted above, retention of sample by the filter device excludes the retained portion of the sample from analysis. Thus, it may not be possible to use the Mar filtering device when only a small amount of unfiltered sample is available. Furthermore, providing space for a thick filter medium and its lengthy filtration path can cause an increase in the overall size of the filtration device.
The thick filter of Mar also presents a small surface area to the liquid on the upstream side of the filter. Therefore, more time or more pressure is required to force the entire sample through the filter, and the process is less efficient. It thus will be seen that although Mar provides filtration of sample during transfer of the sample to an analyzer, many problems remain affecting sample loss, sample protection, and filtration efficiency. Finally, the Mar device requires a gas-tight seal between the tip and plunger during proper operation. For example, if these parts are not precisely aligned, a leak may occur during sample transfer. Therefore, precision manufacturing and operation is required to obtain a gas-tight seal.
Thus, there has been a need in the art for a system of pre-filtering samples to be delivered automatically to an analyzer, which does not require a separate filtering step or manual handling of the sample, which does not require a thick filter element for structural integrity, which loses a minimum amount of sample within the filter medium, which filters liquid quickly without developing a large pressure build-up across the filter element, which does not require a gas-tight seal in use, and which consists of inexpensive, disposable components.